Self-Assessment for UK tradespeople: what to include, deadlines, common mistakes

Self-Assessment is the SA100 tax return every sole trader fills in once a year to tell HMRC how much they earned, what they spent on business, and therefore how much tax they owe. MTD for Income Tax (from April 2026) is phasing it out in favour of quarterly digital updates — but for most tradespeople under £30k, SA100 is still the annual job.

The key dates

• 6 April — tax year begins
• 5 April (next year) — tax year ends
• 31 July — payment on account #1 due (half of last year’s tax bill)
• 31 October — paper return deadline (don’t bother, file online)
• 31 January — online return deadline AND balancing payment AND first payment on account for the new year

Miss 31 January by one day: £100 fine. Three months late: £10/day on top for 90 days. Miss it by a year: minimum £1,600 in penalties.

What income to declare

• Every invoice you issued that was paid in the tax year
• Cash jobs — yes, those too
• CIS deductions — declared gross, with the deductions then offset against your tax owed
• Any side income (selling tools, referral fees, YouTube, whatever)

What you can claim as expenses

Full list on gov.uk, but for tradespeople the common ones are:

• Materials. Bricks, pipes, cables, fittings, consumables.
• Subcontractor labour you paid out (with CIS deductions if applicable).
• Tools. Claimable either as a full expense (if under £200 each) or via capital allowances (if more, like a £2,000 van-mounted compressor).
• Van running costs. Fuel, insurance, tax, MOT, repairs, depreciation — or HMRC mileage at 45p/25p if that’s simpler.
• Workwear. Branded clothing, safety gear, steel-toes. Not generic clothing.
• Home as office — £6/week flat rate or actual proportion if you have a dedicated space.
• Phone and internet — business-use portion.
• Accountant fees and Holdfort subscription.
• Training courses relating to your current trade (not learning a new trade from scratch).
• Insurance. Public liability, professional indemnity, tool cover.
• Trade association fees (Gas Safe, NICEIC, etc.)

Five mistakes that trigger enquiries

1. Expenses that don’t match income. £80k turnover and £78k expenses = suspicious.
2. Round numbers. £5,000 materials, £3,000 tools, £2,000 van — estimated figures set alarms off.
3. Personal purchases claimed as business. A family holiday as "training", a Sky subscription as "advertising".
4. Cash income omitted. HMRC cross-references bank deposits. Small cash-only businesses get inspected.
5. Big year-on-year jumps unexplained. If your expenses double without an obvious reason, expect questions.

How to make this painless

The key is keeping digital records through the year, not rebuilding them in January. Holdfort logs every invoice, every receipt, every mileage claim as you go, then exports a clean CSV you or your accountant file the SA100 from.

See also our receipt rules guide and invoicing basics.

Payments on account explained — the bit that catches new sole traders

If your tax bill for the year is more than £1,000, HMRC wants you to pre-pay most of next year’s bill in two payments on account. Here’s what that looks like:

You filed your first Self Assessment for 2025-26. Your tax owed: £6,000. The bill you receive for 31 January 2027:

• Balancing payment for 2025-26: £6,000
• First payment on account for 2026-27: £3,000 (half the previous year’s total)
• Total due 31 January 2027: £9,000

Then again on 31 July 2027:

• Second payment on account for 2026-27: £3,000

So in the first year of sole-trader tax, you pay 1.5 years’ worth of tax in 7 months. Many new tradespeople aren’t ready for this. Budget for it by setting aside ~30-35% of profit as it lands.

Specific claimable expenses — the tradesperson’s checklist

Often missed by tradespeople doing their own return:

• Your phone as a business expense: if your phone is on a business contract, 100%. If personal contract with business use, claim the business-use percentage.
• Home office: £6/week flat rate (£312/year) without needing to calculate, or the actual proportion of home costs if you have a dedicated office space.
• Trade subscriptions: Gas Safe, NICEIC, FMB membership, Checkatrade fees — all deductible.
• Professional training: courses to maintain/update existing skills (e.g. Part P refresher) are deductible. Courses to learn a new trade from scratch are not.
• Accountant fees: fully deductible.
• Bank charges on business account: fully deductible.
• Insurance: public liability, professional indemnity, tool insurance, van insurance — all deductible.
• Personal protective equipment: steel-toe boots, hi-vis, hard hats, safety glasses, gloves, knee pads — all deductible.
• Workwear: branded company clothing is fully deductible. Plain work clothing (like ordinary jeans that happen to be worn for work) is not, per HMRC guidance.
• Van finance / hire purchase interest: the interest portion is deductible; the capital portion goes through capital allowances.

A sample tax computation

Sole-trader plumber, 2026-27:

• Turnover: £78,000
• Materials and parts: £22,000
• Van + fuel + insurance: £6,000
• Tools + workwear: £900
• Insurance + subscriptions: £800
• Phone + Holdfort + accountant: £1,200
• Training: £300
• Taxable profit: £46,800
• Personal allowance: £12,570 (0%)
• Basic rate 20% on £34,230: £6,846
• Class 2 NI (weekly): £179
• Class 4 NI on profits above £12,570: £2,600
• Total tax + NI owed: £9,625

That’s the number to have ready in your savings account every January. Plus the payment on account 7 months later.